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# Pressure Relief Valve For Fire Water System

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## Pressure Relief Valve For Fire Water System

(OP)
hi all,

i need to perform sizing calculation of Pressure Relief Valve (PRV) for Fire Water System. According to NFPA 20, if 121% of shuttoff pressure + max static pressure exceeds the rated system components, PRV shall be installed. in my case, the pressure is above the max allowable pressure for the fittings (around 250 psig), so i shall design the PRV.

i once design the PSV for gas application (Blocked discharge / fire governing case ). but i have confussion to size the PRV for my fire water system (1000 gpm). should i design PRV based on API 520 ? or NFPA 20 ? because in table 2-20 NFPA 20 (1999 edition). it is stated that the minimum relief valve size for 1000 gpm is 4 inch. i dont understand, because usually i calculate the area of orifice which is in square inch, not in inch. could you please guide me ?

if i decide to calculate based in API 520 , i need to determine the back pressure (which is pressure drop from my fire water pump to fire hydrant , isn't it ? ) i need a balanced below PRV because if my back pressure is 85 psig, it is less than 30% of my set presure (250 psig). am i right ?

please enlighten me :)

### RE: Pressure Relief Valve For Fire Water System

You might want to check the latest edition, your copy is pretty old.

< it is stated that the minimum relief valve size for 1000 gpm is 4 inch >

The 4"is the body size, I would assume the inlet flange but I haven't looked in the NPFA 20. If there isn't a sizing method in NFPA 20, then I would use the formulas in API 520. The basic concept of sizing a relief valve is the same but NPFA could have some specific "tweaks".

<, i need to determine the back pressure (which is pressure drop from my fire water pump to fire hydrant , isn't it ? ) >

No, it's the pressure needed to flow the capacity of the PSV from its outlet flange through the outlet piping to whatever the final destination is. Backpressure is explained in API 520 or API 521.

How are you coming up with the 1000 gpm flow?

### RE: Pressure Relief Valve For Fire Water System

With regard to NFPA-20, I recall that the relief valve size was determined based on a quantity of flow. All well and good (maybe) in the past, but this was based on a pressure of (I think) 70 PSIG and 25 % overpressure (as was liquid sizing in the past prior to ASME introduced 10 %). Basically, going by NFPA-20 you will select an oversized RV. No good on liquid service resulting in chatter and failure. Best to size per API-520 10 % certified overpressure method for stable operation. You may get varying sizes from each manufacturer due to larger differences in certified flow co efficient for liquid trim RV's.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

### RE: Pressure Relief Valve For Fire Water System

NFPA 20 is not requiring a Pressure Relief Valve. It is requiring a Pressure Reducing Valve. Your piping system is operating at pressure > 175 PSIG. So if your pipe and fitting are not rated for the higher pressure service, a Pressure Reducing Valve is required.

### RE: Pressure Relief Valve For Fire Water System

Fire water piping is exempt from API 520 as it is not considered a coded pressure vessel. I would highly recommend consulting an engineering consultant firm that specializes in PSVs. Installing a PSV willy nilly can lead to more problem than benefit.

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